The present invention relates to an electric actuator for controlling opening and closing of a portal valve in power equipments or chemical plants and, particularly, a valve stem driving apparatus for detecting wear rate of a valve stem and/or a stem nut to increase reliability of switch control.
As illustrated in FIG. 2, in a known large portal valve 101, an electric actuator 103 which comprises a screw jack 102 controls opening and closing of a valve body 104.
In order to control stopping at fully opened or closed position of the valve body 104 which is operated by the electric actuator 103 to open and close, a position detecting limit switch 107 which comprises a multistage gear mechanism 106 connected to a motor 105 for the electric actuator 103 detects exact stopping positions corresponding to fully opened and closed positions, so that an electromagnetic switch 110 turns on and off a power source 109 via a motor control circuit 108 including the limit switch.
Limiting operation stop positions of the limit switch 107 are normally adjusted in periodical inspection every some years after initial determination, and operation has been performed at the inspection till next inspection.
The multistage gear mechanism 106 for actuating the limit switch 107 is connected to an opening-degree indicator 111 which displays an opening degree of the valve.
Valve is monitored by the limit switch 107 and the mechanical opening-degree indicator 111, both being indirect detecting means on an input shaft of a screw jack 102.
Owing to errors in the points of contact of the limit switch 107 during operation, wear in power transmission system and gaps by damage, there is difference in opening degree corresponding to contact signal of the limit switch or order of mechanical opening degree indicator, thereby making normal operation and observation of valve impossible. If still used, total pipe line control system would be broken.
To increase reliability, it would be useful in indistrial plants if it is monitored whether or not a valve is controlled in a normal position without position detecting mechanism on a valve driving side.
The adjustment position of the limit switch is determined by measuring stem position with a scale such as a depth gauge after removal of a stem cover 112, but direct observation of valve position would make switching means for the stem cover unnecessary and inspection time shortened. It would be useful for decreasing suffering from radioactivity in a nuclear generator.